1. Field of the Invention
This invention pertains to an exposure unit for imaging photosensitive materials, particularly photosensitive materials used to form flexographic printing plates, and to a method for uniformly exposing photosensitive materials.
2. Description of the Prior Art
Flexographic printing plates are elastomeric plates which generally have a relief depth in the range of 10 to 150 mils. Processes for producing flexographic printing plates from a photosensitive element are well known in the art. Typically, these processes include (1) exposing imagewise a photopolymerizable element to actinic radiation to produce a difference in solubility between the exposed and non-exposed areas, (2) removing the more soluble areas with a developer solvent, and (3) drying the resulting plate. The process usually also includes (4) exposing to a "backflash" exposure, (5) detackifying and (6) post-exposing overall.
The three exposure steps, i.e., backflash, imagewise exposure, and post-exposure, can be accomplished using actinic radiation from a variety of sources such as, for example, lamps. Exposure times vary from a few seconds to a few minutes depending upon the output of the lamps, distance from the lamps, relief depth desired and the thickness of the plate.
Exposure units which are currently used in the production of flexographic printing plates have several disadvantages. The exposure units require long exposure times which adversely affects productivity. Also, they are not capable of consistently providing a uniform distribution of radiation, particularly to large plates. In addition, the current exposure units are not capable of creating the type of relief profile required for very high-resolution printing.